This invention relates to power saving operation of a cellular telephone handling a control channel and, more particularly, to the use of a digital phase locked loop with inhibition of phase adjustment during receiver power-down intervals.
Radiotelephones and particularly cellular telephones may be operated in a standby mode for conservation of electric power, particularly power provided by a battery. During the standby mode, the radiotelelephone must continuously receive and monitor signaling data sent by a base station via a forward control channel (FOCC). The signaling data employs a signaling format as is described, for example, in AMPS (EIA 553), DAMPS, CDMA and TACS specifications, and with a prescribed signal frame structure.
Radiotelephones are operated often on battery power. In order to provide a lengthy interval of operation without replacement or recharging of the battery, it is advantageous to construct and operate the electric circuits of the radiotelephone in a manner which conserves the utilization of electric power. An important example of such power conservation is found in the receiving circuitry employed in the FOCC. Examples of such power conservation are found in the following U.S. patents, namely, L. J. Harte, U.S. Pat. No. 5,224,152, T. J. Auchter, U.S. Pat. No. 5,175,874, E. J. Addeo, U.S. Pat. No. 4,029,900, and M. A. S El-Banna, U.S. Pat. No. 4,592,076. With respect to the conservation methods disclosed in the foregoing patents, there is a common problem in that, each time the receiver is turned off, a new synchronization is to be achieved from the Bit Sync and Word Sync patterns preceding the next frame to be received. If bit errors appear in these patterns, synchronization is not achieved, and the next frame is lost. One may implement pattern detection which detects the pattern with one or more bit errors, but this would increase a possibility of finding the pattern in a data field or from noise with a consequent erroneous frame synchronization.
A typical situation wherein bit errors arrive in a received signal, specifically in the synchronization patterns, is the situation wherein a radiotelephone is moving among various objects which reflect the signal and introduce multipath propagation of the signal. This causes short-term fading (Rayleigh fading) wherein, during fade peaks the received signal is very weak. In the foregoing methods of power conservation, there is a loss in synchronism for received data each time that the receiver is turned off and, accordingly, such method of power conservation may be referred to as a non-synchronous mode (NSM). Due to the loss of synchronism, the synchronism has to be reestablished at the beginning of each signal frame by a detection of the synch patterns.
A further disadvantage to the NSM mode of power conservation is that the receiver must be turned on with a significant advance in time, well before the synch pattern appears so that the RF (radio frequency) parts of the radiotelephone circuitry have sufficient time to adjust to the channel, and wherein the baseband parts of the radiotelelephone circuitry have sufficient time to settle. By way of example in the settling of the baseband part of the circuitry, the circuitry normally employs a digital phase locked loop (DPLL) for synchronization to the bits of the frame signal before a synch pattern can be detected.
As a result, optimum power saving is not achieved because the receiver circuitry must be energized well before the occurrence of the synch patterns.
Yet a third aspect to the foregoing problem may be understood with respect to the signal formatting wherein the FOCC signal has both an A portion and a B portion wherein coded words in each of the portions are repeated. The repeats of the words of the A portion (A1, A2, . . . A5) and the repeats of the word of the B portion (B1, B2 . . . B5) are interleaved. Therefore, if radiotelephone is to be responsive to the B portion of the FOCC, then it is necessary to view also the A portion, although not needed, with a resulting wastage of power. Similarly, it is apparent that during reception of the B portion of the FOCC channel, the receiver is turned off later than in the case of a radiotelephone programmed to respond to the A portion. This results in less power saving for the B portion than for the A portion of the FOCC.
In a fourth aspect of the foregoing problem, it is noted that the microcontroller of the radiotelephone performs certain operations for each received frame of the control signal. Thus, if the microcontroller is operating in a standby mode, it must be interrupted and activated to operate at least once for each received frame. The foregoing problems arise because of the nonsynchronous mode of operation of the energy conservation procedures and circuitry.